Is the attack modifier of green-flame blade or booming blade based on your spellcasting ability, or your weapon attack modifier?

For example, if my high-Charisma, low-Strength sorcerer were to use green-flame blade and make an attack roll with a longsword, would the modifier be his high Charisma mod, his low Strength mod, or his/the DM's choice of either one?

2 Answers
2

Use the ability appropriate for your weapon

The spell specifies no exception to the normal rules for melee weapon attacks, so the attack uses normal rules: you use the Strength modifier, or if you're using a Finesse weapon, optionally your Dexterity modifier. Using other ability modifiers may become possible through magic (eg. Shillelagh).

You might include the (what I think is the obvious) "why" these spells are written as they are which sort of breaks the standard mold for damage dealing spells... which is that, in addition to the spell's effects, the target takes the normal weapon damage. Making these a touch more powerful coupled with the risk of melee range.
– SlagmothDec 6 at 13:30

1

The term "melee spell attack" is the term that would appear if it were using the spellcasting modifier.
– NachtDec 6 at 21:37

The spell calls for a melee weapon attack and thus, without it specifically calling for something else, follows the normal attack rules and not the spell attack rules. For a non-finesse weapon that will use your Strength mod plus a proficiency bonus if applicable.